Method and apparatus for dissipative clamping of an electrical circuit

ABSTRACT

Dissipative clamping apparatuses and methods for electrical circuits. In one aspect of the invention, In one aspect of the invention, a method includes switching a power supply input on an energy transfer element, regulating a power supply output by switching the power supply input on the energy transfer element, clamping a voltage on the energy transfer element to a clamp voltage and varying the clamp voltage in response to the power supply input. In another aspect, an electrical circuit includes a dissipative clamp circuit coupled to an input of the electrical circuit. An inductive element is coupled between the dissipative clamp circuit and an output of the electrical circuit. A switch is coupled in series with the inductive element. The dissipative clamp circuit is coupled to provide a clamp voltage across the inductive element, the clamp voltage is provided by the dissipative clamp circuit responsive to conditions at the input of the electrical circuit, the dissipative clamp circuit is coupled to maintain a voltage across the switch below a switch voltage limit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to electrical circuitsand, more specifically, the present invention relates to electricalcircuit clamping.

[0003] 2. Background Information

[0004] Electronic devices use power to operate. Switched mode powersupplies are commonly used due to their high efficiency and good outputregulation to power many of today's electronic devices. In a knownswitched mode power supply, a low frequency (e.g. 50 or 60 Hz mainsfrequency), high voltage alternating current (AC) is converted to highfrequency (e.g. 30 to 300 kHz) AC, using a switched mode power supplycontrol circuit. This high frequency, high voltage AC is applied to atransformer to transform the voltage, usually to a lower voltage, and toprovide safety isolation. The output of the transformer is rectified toprovide a regulated direct current (DC) output, which may be used topower an electronic device. The switched mode power supply controlcircuit usually provides output regulation by sensing the output andcontrolling it in a closed loop.

[0005] To illustrate, FIG. 1 is a schematic of a known forward powerconverter 101. A switch Q1 103 turns on and off in response to a control105 to provide a regulated DC output voltage V_(OUT) 129 from anunregulated DC input voltage V_(IN) 127. In one embodiment, control 105and switch Q1 103 are included in a switching regulator, which may beused to regulate the output voltage V_(OUT) 129. This topology is wellknown and its operation is well documented.

[0006] Every forward converter must have a way to set the voltage on theprimary winding 107 of the transformer 109 during the time when theswitch Q1 103 is off. A popular way to set the voltage is with a clampnetwork 111 connected across the primary winding 107. The known clampnetwork 111 illustrated in FIG. 1 includes a resistor 113, a capacitor115 and a diode 117 and absorbs and dissipates parasitic energy from thetransformer 109 that is not delivered to the load 119 nor returned tothe input 121. The balance of energy into the clamp network 111 throughdiodes 117 and energy dissipated in 113 determines a clamp voltageV_(CLAMP) 123 that is necessary prevent saturation of the transformer109.

[0007]FIG. 2 shows with idealized waveforms how the voltage V_(SWITCH)125 on switch Q1 103 is related to the input voltage V_(IN) 127 and theclamp voltage V_(CLAMP) 123. The clamp voltage V_(CLAMP) 123 must behigh enough to prevent saturation of the transformer 109, but low enoughto keep the voltage V_(SWITCH) 125 below the breakdown voltage of switchQ1 103.

[0008]FIG. 3 shows the relationship between V_(CLAMP) 123 and V_(IN) 127in a known power supply. As the input voltage V_(IN) 127 changes, theclamp voltage V_(CLAMP) 123 must be confined between the two boundariesshown in FIG. 3. The maximum voltage boundary is a straight linedetermined by the breakdown voltage of switch Q1 103. The minimumvoltage boundary is a curved line determined by the voltage necessary tokeep the transformer 109 from saturation.

[0009]FIG. 3 shows how the clamp voltage V_(CLAMP) 123 behaves with anRCD network, such as that illustrated in clamp network 111 of FIG. 1.When the power converter 101 operates in continuous conduction mode, theclamp voltage V_(CLAMP) 123 stays substantially constant in response tochanges in V_(IN) 127 at given load. The presence of leakage inductancein the transformer 109 causes the clamp voltage V_(CLAMP) 123 to changewith load 119. It is higher for greater current and lower for lesscurrent. The result is a restricted range of permissible input voltageV_(IN) 127 that is shown in the shaded region of FIG. 3.

SUMMARY OF THE INVENTION

[0010] Dissipative clamping methods and apparatuses are disclosed. Inone aspect of the invention, a method includes switching a power supplyinput on an energy transfer element, regulating a power supply output byswitching the power supply input on the energy transfer element,clamping a voltage on the energy transfer element to a clamp voltage andvarying the clamp voltage in response to the power supply input.Additional features and benefits of the present invention will becomeapparent from the detailed description, figures and claims set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention detailed illustrated by way of example andnot limitation in the accompanying figures.

[0012]FIG. 1 is a schematic diagram illustrating a known forwardconverter power supply.

[0013]FIG. 2 is a timing diagram illustrating how the voltage on theswitch is related to the input voltage and the clamp voltage in a knownpower supply.

[0014]FIG. 3 is a diagram illustrating the relationship between theclamp voltage and the input voltage in a known power supply

[0015]FIG. 4 is a block diagram illustrating one embodiment of thegeneral elements of a dissipative clamp network in accordance with theteachings of the present invention.

[0016]FIG. 5 is a diagram illustrating one embodiment of therelationship between the clamp voltage and the input voltage inaccordance with the teachings of the present invention.

[0017]FIG. 6 is a schematic diagram illustrating one embodiment of apower supply using a dissipative clamp network in accordance with theteachings of the present invention.

DETAILED DESCRIPTION

[0018] Embodiments of methods and apparatuses for dissipatively clampingan electrical circuit such as a power supply regulator are disclosed. Inthe following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

[0019] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures or characteristics may be combined in any suitable manner inone or more embodiments.

[0020] As an overview, FIG. 4 shows the general elements of oneembodiment of a dissipative clamp network 411 in an electrical circuit,such as for example a power supply 401, in accordance with the teachingsof the present invention. As shown, an input voltage V_(IN) 427 isreceived at an input 421. A clamp network 411 is used to clamp thevoltage V_(CLAMP) 423 across the primary winding 407 of a transformer409. A switch 403 is coupled to primary winding 407 to drive primarywinding 407 in response to a control circuit (not shown). It isappreciated that transformer 409 is an inductive element and may bereferred to as an energy transfer element or the like. A clamp diodeD_(CLAMP) 437 provides a unidirectional path for the energy from theprimary winding 407 of the transformer 409 to enter the clamp network411. The energy is held by an energy storage element 435 and is lostthrough a dissipative element 433. In one embodiment, the dissipativeelement 433 is programmed by a signal S₁ 439 from a sensing network 431.The sensing network 431 produces the programming signal S₁ 439 frommeasurements of the input voltage V_(IN) 427, the voltage on the energystorage element 435 and a reference voltage V_(REF) 441 received by thesensing network 431. Thus, in one embodiment, energy stored in theleakage inductance of transformer 409 is dissipated in response to theinput voltage V_(IN) 427.

[0021] In one embodiment, the dissipative element 433 is adapted inaccordance with the teachings of the present invention, which can beviewed as having the effect of changing the value of the resistor 113 inthe RCD clamp network 111 of FIG. 1. The control from programming signalS₁ 439 from sensing network 431 adjusts the energy balance to maintain adesired locus of clamp voltage over an extended range of input voltageas illustrated in FIG. 5. As shown, in one embodiment the clamp voltageV_(CLAMP) 423 is varied substantially inversely linearly with respect tothe input voltage V_(IN) 427 in accordance with the teachings of thepresent invention. Thus, in one embodiment, the clamp voltage V_(CLAMP)423 is varied substantially independent of the power supply outputand/or leakage inductance of transformer 409.

[0022] With the variation in clamp voltage V_(CLAMP) 423 as shown, therange of input voltages for V_(IN) 427 is increased in accordance withthe teachings of the present invention. Indeed, various embodiments ofthe present invention allow operation over an extended range of inputvoltage for V_(IN) 427 while maintaining the clamp voltage V_(CLAMP) 423at a high value within the minimum and maximum boundaries as shown inFIG. 5. The higher voltages made possible by a variable clamp voltageV_(CLAMP) 423, such as illustrated in FIG. 5, allows the use ofparasitic capacitance in the primary winding 407 and secondary windings443 to process some of the energy that otherwise would be dissipated inthe clamp circuit 411.

[0023]FIG. 6 is a schematic illustrating one embodiment of an electricalcircuit such as for example a power supply 601 utilizing a dissipativeclamp network 611 in accordance with the teachings of the presentinvention. As shown in the depicted embodiment, diode D3 637 providesthe unidirectional path for energy from the primary winding 607 of thetransformer 609 to enter the network 611 and capacitor C2 635 is theenergy storage element of the clamp network 611. Zener diode VR1 645 andcapacitor C3 647 make a stable reference voltage V_(REF) 641. In oneembodiment, an N-channel metal oxide semiconductor (MOS) transistor Q2is the principal dissipative element 633. In another embodiment, it isappreciated that other types of dissipative elements could be used inplace of an N-channel MOS transistor such as for example p-channel MOStransistor, a bipolar transistor or the like or other future arisingtechnology performing the function. In one embodiment, the sensingnetwork in power supply 601 includes the connection of resistors R1 649,R2 651, R3 653 and R4 655 with transistor Q3 657. The voltage on thegate of transistor Q2 633 is the programming signal S₁ 639 that adaptsthe dissipation to achieve the desired characteristic of operation.

[0024] In one embodiment, resistors R2 651 and R4 655 form a voltagedivider that applies a scaled value of the sum of the input voltageV_(IN) 627 received at input 621 and the reference voltage V_(REF) 641from Zener diode VR1 645 to the base of transistor Q3 657. The currentflowing through R3 653 is proportional to the difference in voltagebetween the base of transistor 657 Q3 and the input voltage V_(IN) 627.The result is a current in the collector of transistor Q3 657 thatdecreases substantially linearly with increasing input voltage V_(IN)627. The collector current in transistor Q3 657 produces a voltage dropthrough resistor R1 649 such that the voltage, or programming signal S₁639, on the gate of transistor Q2 633 is proportional to the weightedsum of the clamp voltage V_(CLAMP) 623 and the input voltage V_(IN) 627.The gate voltage on the gate of transistor Q2 633 controls the currentin the dissipative element transistor Q2 633 to adjust the clamp voltageV_(CLAMP) 623 at a desired value for a given V_(IN) 627.

[0025] A first order analysis using reasonable engineeringapproximations reveals that the behavior of the circuit of power supply601 is described by the expression$V_{CLAMP} = {{V_{REF}\left( {1 + \frac{{R1} \cdot {R4}}{{R3}\left( {{R2} + {R4}} \right)}} \right)} - {V_{IN}\left( {2 - \frac{{R1} \cdot {R2}}{{R3}\left( {{R2} + {R4}} \right)}} \right)}}$

[0026] that describes a substantially straight line on the graph ofV_(CLAMP) versus V_(IN), as shown in FIG. 5. An engineer can selectvalues for resistances R1 649, R2 651, R3 653 and R4 655 along withV_(REF) 641 to achieve the locus of desired operation as illustrated inFIG. 5.

[0027]FIG. 7 is a schematic illustrating one embodiment of an electricalcircuit such as for example a power supply 701 utilizing a dissipativeclamp network 711 in accordance with the teachings of the presentinvention. As shown in the depicted embodiment, diode D3 737 providesthe unidirectional path for energy from the primary winding 707 of thetransformer 709 to enter the network 711. Zener diode VR1 745 makes astable reference voltage V_(REF) 741 relative to the circuit inputnegative rail of input 721. In one embodiment, a bipolar PNP transistorQ2 733 is the principal dissipative element. In another embodiment, itis appreciated that other types of dissipative elements could be used inplace of a bipolar PNP transistor 733 such as for example a P channelMOSFET transistor. Resistor R1 753 is an optional additional dissipativeelement allowing the dissipated energy to be split between the bipolartransistor 733 and resistor R1 753. The energy is held by an energystorage element capacitor 735 and is lost through a dissipative elementstransistor 733 and resistor 753.

[0028] In operation, the sum of the voltages V_(IN) 727 across the input721 and V_(CLAMP) 723 across capacitor 735 is substantially constant.Thus, when V_(IN) 727 is relatively low, V_(CLAMP) 723 is relativelyhigh. Conversely, when V_(IN) 727 is relatively high, V_(CLAMP) 723 isrelatively low. Accordingly, V_(CLAMP) 723 is responsive to V_(IN) 727received at input 721. Since the reference voltage V_(REF) 741 providedby zener diode VR1 745 is relative to the circuit input 721 negativerail, the operation of the clamp network 711 shown in FIG. 7 provides aclamp that limits V_(CLAMP) 723 across capacitor 735 to the locus ofdesired operation shown in FIG. 5. In another embodiment is itappreciated that zener diode VR1 745 reference voltage V_(REF) 741 couldbe achieved with several lower voltage zener diodes in series.

[0029] It is appreciated that in the illustrated embodiment, transistor733 in combination with resistor 753 and diode 745 embody a sensingnetwork to sense V_(IN) 727 and thereby regulate the voltage acrosscapacitor 735 such that the sum of V_(IN) 727 and V_(CLAMP) 723 remainsubstantially constant during circuit operation. Accordingly, thevoltage V_(SWITCH) 725 across power switch Q1 703 is maintained below avoltage limit of power switch Q1 703 in accordance with the teachings ofthe present invention.

[0030] In the foregoing detailed description, the method and apparatusof the present invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. An electrical circuit, comprising: a dissipativeclamp circuit coupled to an input of the electrical circuit; aninductive element coupled between the dissipative clamp circuit and anoutput of the electrical circuit; and a switch coupled in series withthe inductive element; the dissipative clamp circuit coupled to providea clamp voltage across the inductive element, the clamp voltage providedby the dissipative clamp circuit responsive to conditions at the inputof the electrical circuit, the dissipative clamp circuit coupled tomaintain a voltage across the switch below a switch voltage limit. 2.The electrical circuit of claim 1 wherein the dissipative clamp circuitis coupled to be responsive to conditions at the output of theelectrical circuit.
 3. The electrical circuit of claim 1 wherein theelectrical circuit is a power conversion circuit.
 4. The electricalcircuit of claim 3 wherein the power conversion circuit is a forwardconverter power conversion circuit.
 5. The electrical circuit of claim 1wherein the inductive element comprises a winding of a transformer. 6.The electrical circuit of claim 1 wherein the switch comprises a firsttransistor.
 7. The electrical circuit of claim 6 wherein the firsttransistor comprises a first bipolar transistor.
 8. The electricalcircuit of claim 6 wherein the first transistor comprises a first metaloxide semiconductor (MOS) transistor.
 9. The electrical circuit of claim1 wherein the dissipative clamp circuit comprises a second transistorcoupled to the inductive element to dissipate energy stored in theinductive element.
 10. The electrical circuit of claim 9 wherein thesecond transistor comprises a second bipolar transistor.
 11. Theelectrical circuit of claim 9 wherein the second transistor comprises asecond metal oxide semiconductor (MOS) transistor.
 12. The electricalcircuit of claim 1 wherein the input of the electrical circuit iscoupled to receive an input voltage.
 13. The electrical circuit of claim12 wherein the dissipative circuit is coupled to be responsive tovarying voltage conditions at the input of the electrical circuit. 14.The electrical circuit of claim 12 wherein the input of the electricalcircuit is coupled to receive the input voltage from a rectifier coupledto rectify an alternating current (AC) line voltage.
 15. The electricalcircuit of claim 12 wherein the dissipative circuit is coupled to beresponsive to a varying amount of energy being clamped across theinductive element of the electrical circuit.
 16. The electrical circuitof claim 15 wherein the amount of energy being clamped across theinductive element varies in response to a varying peak current in theinductive element.
 17. The electrical circuit of claim 16 wherein theoutput of the electrical circuit is coupled to a load, the varying peakcurrent in the inductive element to vary in response to changes in theload coupled to the output of the electrical circuit.
 18. The electricalcircuit of claim 16 wherein the varying peak current in the inductiveelement is coupled to vary in response to a soft start period of acontrol of the switch.
 19. The electrical circuit of claim 1 furthercomprising a second input coupled to the switch, wherein switching ofthe switch is responsive to the second input of the electrical circuit.20. The electrical circuit of claim 19 wherein the clamp voltageprovided by the dissipative clamp circuit is further responsive toconditions at the second input of the electrical circuit.
 21. Theelectrical circuit of claim 1 further comprising a second output coupledto the inductive element, wherein the clamp voltage provided by thedissipative clamp circuit is further responsive to conditions at thesecond output of the electrical circuit.
 22. A power supply, comprising:an energy transfer element having an energy transfer element input andan energy transfer element output coupled to an output of the powersupply; a switching regulator circuit including a power switch coupledto the energy transfer element input, and a control circuit coupled tothe power switch and the output of the power supply, the control circuitcoupled to switch the power switch to regulate the output of the powersupply; and a dissipative clamp circuit coupled to the energy transferelement input, the dissipative clamp circuit coupled to a power supplyinput to receive an input voltage, the dissipative clamp circuitincluding: a sensing network coupled to the power supply input to sensethe input voltage; a dissipative element coupled to the sensing networkand coupled to the energy transfer element; an energy storage elementcoupled to the energy transfer element and the dissipative element; anda first diode coupled between the power switch and the dissipativeelement and the energy storage element.
 23. The power supply of claim 22wherein the energy storage element comprises a capacitor coupled to theenergy transfer element input and the first diode.
 24. The power supplyof claim 22 wherein the dissipative element comprises a first transistorcoupled to the energy storage element, the first transistor coupled todissipate energy in the energy storage element in response to a signalreceived from the sensing network.
 25. The power supply of claim 22wherein the sensing network comprises: a voltage divider circuit coupledto the reference voltage circuit to provide a scaled voltage responsiveto a reference voltage added to the input voltage; and a secondtransistor coupled to the dissipative element and coupled to the voltagedivider, the second transistor coupled to provide a current that iscoupled to decrease linearly with increasing input voltage.
 26. Thepower supply of claim 25 wherein the reference voltage is provided by areference voltage circuit coupled to the power supply input, thereference voltage circuit including a zener diode coupled between thevoltage divider circuit and the power supply input, the referencevoltage circuit further including a second capacitor coupled between thevoltage divider circuit and the power supply input.
 27. A method,comprising: switching a power supply input on an energy transferelement; regulating a power supply output by switching the power supplyinput on the energy transfer element; clamping a voltage on the energytransfer element to a clamp voltage; and varying the clamp voltage inresponse to the power supply input.
 28. The method of claim 27 whereinthe varying of the clamp voltage is substantially independent of thepower supply output.
 29. The method of claim 28 wherein the varying ofthe clamp voltage is further substantially independent of leakageinductance of the energy transfer element.
 30. The method of claim 27wherein clamping the voltage on the energy transfer element comprisesdissipating energy stored in leakage inductance of the energy transferelement in response to the power supply input.
 31. The method of claim30 wherein varying the clamp voltage comprises varying the clamp voltagesubstantially inversely linearly with respect to the power supply input.